The present invention relates generally to a crystal puller and method for growing monocrystalline silicon ingots, and more particularly to such a crystal puller and method in which a crucible having a high heat transmittance is used to contain molten silicon during the growth of monocrystalline silicon ingots in the crystal puller.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (xe2x80x9cCzxe2x80x9d) method. The growth of the crystal is most commonly carried out in a crystal pulling furnace. In this method, polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by extraction via a crystal puller. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has a generally constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
Crucibles used in conventional crystal pullers are commonly constructed of quartz because of its purity, temperature stability and chemical resistance. The side walls of conventional crucibles are substantially xe2x80x9copaque,xe2x80x9d or more accurately xe2x80x9ctranslucent,xe2x80x9d throughout their thickness as a result of having a high density of bubbles therein. These bubbles are grown into the crucible wall as a result of conventional manufacturing processes in which quartz particles are heated and fused together in a relatively short time period (otherwise commonly referred to as arc fusing). For example, conventional translucent quartz crucibles are typically produced by a process in which quartz powder is introduced into a mold to form a layer along the inner surface of the mold. The quartz powder is then heated and fused together at the inner surface thereof while the mold is rotated to produce a quartz crucible.
Crucibles manufactured in this manner have a relatively high bubble content, i.e., a relatively high density of bubbles or pockets of gas contained in the crucible side wall near the radially outer half of the wall thickness. Such crucibles typically contain bubbles ranging from about 50-200 microns in diameter, with the average bubble having a diameter of about 100 microns. There are approximately 70,000 bubbles/cm3 in translucent crucibles. These quartz crucibles are advantageous in that they have high strength and are relatively easy to form in large sizes. For these reasons, translucent quartz crucibles are widely utilized.
However, quartz crucibles having a high bubble content, such as the conventional translucent crucibles described above as having a high bubble density throughout the thickness of the crucible side wall, are not without drawbacks. Prolonged exposure, e.g., up to 100 hours or more, of the inner surface of the crucible side wall to the high temperature silicon melt contained therein during crystal growth results in reaction of the silicon melt with the quartz crucible and leads to dissolution of the inner surface of the crucible side wall. This dissolution of the inner surface exposes bubbles in the crucible side wall to the molten silicon. As a result, the bubbles rupture, releasing gases inside the bubbles into the melt as well as quartz particles from the crucible side wall. Exposed pits in the inner surface of the crucible resulting from the ruptured bubbles are also known to promote the formation of crystobalite on the roughened inner surface. The crystobalite can become separated from the crucible and float in the melt. This in turn is a particle that can be slow to dissolve.
Particulates in the melt can come into contact with the growing crystal at the melt interface and be incorporated into the crystal structure. When this happens, a resulting loss of zero dislocation structure in the crystal can occur which will lead to a decreased throughput of crystalline ingots. One measurement of the loss of throughput associated with the use of quartz crucibles is the percentage of wafers manufactured from a crystal ingot which have at least one Large Light Point Defect (LLPD""s). A Light Point Defect is a light scattering event off of a polished silicon wafer surface which can be registered by an inspection tool and is the result of a localized topographical deviation from the nominally planar silicon surface. In other words, the Light Point Defect is the result of a particle or pit on the wafer surface that causes an increase in light scattering intensity relative to that of the surrounding wafer surface. Bubbles which cavitate from the crucible can become entrained in the melt and subsequently attach to the liquid/solid interface. This essentially results in a grown in bubble in the crystal. Consequently, when the crystal is sliced into wafers, the cut can progress through the bubble, with a resulting pit on the wafer surface. Light Point Defects that have a large scattering potential, such as pits corresponding to a size of at least 10 microns or more in diameter, are classified as Large Light Point Defects.
To this end, it is conventional in the art to manufacture a translucent, multi-layer quartz crucible comprising an inner layer having a reduced bubble size and/or density and an outer layer having a relatively high bubble density. For example, U.S. Pat. No. 4,632,686 (Brown et al.) discloses a method of manufacturing quartz glass crucibles to have a xe2x80x9clow bubble content.xe2x80x9d This method comprises applying a vacuum pressure to the outer surface of the crucible during heating and fusion of the quartz powder. However, because the bubbles drawn from the inner layer of the crucible encounter substantial resistance in passing through the outer layer, the outer layer of the crucible has a relatively high bubble size and density. An inspection of currently available crucibles manufactured according to this or similar applied vacuum processes reveals that the xe2x80x9clow bubble contentxe2x80x9d provided by such a method includes bubbles having an average size of about 150 microns and a bubble density of about 5000 bubbles/cm3.
U.S. Pat. No. 4,935,046 (Uchikawa et al.) discloses another multi-layer quartz glass crucible in which an outer, or base layer is translucent and an inner layer (e.g., 0.3 mm to 3.0 mm) is substantially transparent. The substantially transparent inner layer is said to be free of bubbles having a diameter greater than 50 microns and bubbles of a diameter between 20 microns and 50 microns do not exist in a concentration of more than 10/cm2. The stated bubble density is given as a surface area measurement without regard to the thickness of the crucible. A volumetric density expressed in bubbles/cm3 would be considerably higher. In any event, when the inner layer of the crucible is dissolved into the melt, particulate contamination of the melt resulting from ruptured bubbles and the growth of crystobalites is reduced. Providing the high bubble density outer layer is said to provide a more uniform heating throughout the inner surface of the crucible so that the crystal pulling operation can be carried out with an improved stability. According to Uchikawa et al., if the bubble concentration in the outer layer is too low, a sufficient diffusion of heat radiated by the crucible heater will not be obtained.
Conventional crucibles such as those disclosed above have had limited success in eliminating all quartz crucible related crystal defects. Due to the long duration of crystal pulling required to produce large diameter crystals, the low bubble content inner layer of the crucible of Uchikawa et al. may be dissolved before crystal growth is complete and the higher bubble density portion of the crucible may then expose more bubbles to the melt.
Moreover, even if the inner layer is sufficiently thick, the outer layer still contains a high density of bubbles. Contrary to the teachings of Uchikawa et al., it has recently been discovered that the size and density of bubbles present in conventional translucent quartz crucibles suppresses the pull rate at which an ingot can pulled up from the melt without distortion of the growing ingot. The bubbles in the crucible tend to increase thermal resistance against heat transmittance from the crucible heater through the crucible side wall to the molten silicon in the crucible. More particularly, the bubbles diffuse and scatter heat energy from the crucible heater, thereby negatively effecting heat transmittance through the crucible to the molten silicon. The result of the low transmittance of conventional crucibles is that the melt becomes increasingly isothermal, i.e., the melt has a relatively low temperature gradient, which is believed to limit the pull rate at which the ingot can be pulled from the melt without distortion.
Among the several objects and features of the present invention may be noted the provision of a crystal puller and method which increases throughput of monocrystalline silicon ingots grown in the crystal puller; the provision of such a crystal puller and method which allows for increased pull rates without distortion of the ingots; and the provision of such a crystal puller and method which reduces the incidence of Large Light Point Defects in wafers manufactured from ingots grown in the crystal puller.
Generally, a crystal puller of the present invention for growing monocrystalline silicon ingots according to the Czochralski method comprises a housing and a crucible in the housing for containing molten silicon. The crucible includes a side wall having a transmittance of at least about 80% generally throughout a light wavelength range of about 500 to about 2500 nanometers. A pulling mechanism is included for pulling a growing ingot upward from the molten silicon.
A method of the present invention for growing a monocrystalline silicon ingot in a crystal puller of the type used for growing monocrystalline silicon ingots according to the Czochralski method generally comprises charging polycrystalline silicon to a crucible having a side wall. Heat is radiated through the crucible side wall with a transmittance of at least about 80% generally throughout a light wavelength range of about 500 to about 2500 nanometers to melt the polycrystalline silicon for forming a molten silicon melt in the crucible. A seed crystal is brought into contact with the molten silicon in the crucible and a monocrystalline silicon ingot is pulled up from the molten silicon.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.